1. Field of the Invention
The present invention relates to the field of communications, and more particularly, the present invention relates to ultrawide-band impulse communication systems and methods employing subcarriers.
2. Related Art
Designers of radio technology for personal communications devices, medical and military devices, and the like, are currently faced with several development challenges. Low power consumption, reuse of available spectrum, channelization and cost are four of the main issues.
These issues are addressed in part by an emerging, revolutionary technology called impulse radio communications (hereafter called impulse radio). Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989) and U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994), all to Larry W. Fullerton. These patent documents are incorporated herein by reference.
Basic impulse radio transmitters emit short Gaussian monocycle pulses with tightly controlled average pulse-to-pulse interval. Impulse radio systems use pulse position modulation. Pulse position modulation is a form of time modulation in which the value of each instantaneous sample of a modulating signal is caused to modulate the position in time of a pulse.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Spread spectrum systems make use of pseudo-random codes to spread the normally narrowband information signal over a relatively wide band of frequencies. A spread spectrum receiver correlates these signals to retrieve the original information signal. Unlike spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide information bandwidth (information bandwidth, hereafter called bandwidth, is the range of frequencies within which performance, with respect to some characteristics, falls within specific limits). Instead, the pseudo-random code is used for channelization, energy smoothing in the frequency domain, and jamming resistance.
The impulse radio receiver is a homodyne receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. (The baseband signal is the basic information channel for the basic impulse radio communications system, and is also referred to as the information bandwidth.) The data rate of the impulse radio transmission is only a fraction of the periodic timing signal used as a time base. Each data bit time position modulates many pulses of the periodic timing signal. This yields a modulated, coded timing signal that comprises a train of identical pulses for each single data bit. The cross correlator of the impulse radio receiver integrates multiple pulses to recover the transmitted information.
As with all aspects of the electronics field, what is desired are still smaller, lower power and more flexible systems. However, generally accepted principles in continuous wave (CW) radio technology do not readily lend themselves to time domain systems, such as impulse radio.
Descriptions of some of the basic concepts discussed below are found in a number of references, including Robert C. Dixon, Spread Spectrum Systems (John Wiley and Sons, Inc., New York, 1984, 2nd ed.); and Don J. Torrieri, Principles of Military Communication Systems (Artech House, Inc., Dedham Mass., 1982, 3rd ed.).
The impulse radio communications system according to the present invention uses one or more subcarriers to communicate information from an impulse radio transmitter to an impulse radio receiver. Three impulse radio communications system embodiments are described, including: a one channel system, a two channel system and a three or more channel system. Typical radio frequency impulse radio communications system applications include cellular telephones, wireless telephones, wireless PBXs/Local area networks, and the like. The impulse radio communication system is an ultrawide-band time domain system. Operation in the time domain is in accordance with general impulse radio theories discussed below in section II. The use of subcarriers provides impulse radio transmissions added channelization, smoothing and fidelity. Subcarriers of different frequencies or waveforms can be used (simultaneously) to add channelization of impulse radio signals. Thus, an impulse radio link can communicate many independent channels simultaneously by employing different subcarriers for each channel.
There are three impulse radio transmitter embodiments. The first and second transmitter embodiments comprise a subcarrier generator and modulator that uses one or more information signals to modulate a periodic timing signal.
According to the first embodiment, coding of the impulse radio signals is achieved by coding the periodic timing signal before it is time modulated by the modulated subcarrier signal.
According to the second embodiment, coding of the impulse radio signals is achieved by coding a modulated subcarrier signal before it is used to time modulate the periodic timing signal.
The third transmitter embodiment comprises a subcarrier generator and modulator that uses one or more information signals to modulate a periodic timing signal in combination with direct digital modulation of a digital data signal. In this embodiment, the modulated subcarrier signal is used to time modulate the direct digitally modulated signal.
The impulse radio transmitter generally comprises a time base that generates a periodic timing signal. The time base comprises a voltage controlled oscillator, or the like, having sub-nanosecond timing requirements. The periodic timing signal is supplied to a code source and to a code time modulator. The code source comprises a storage device for storing nearly orthogonal pseudo-random noise (PN) codes and means for outputting the PN codes as a code signal. The code source monitors the periodic timing signal to permit the code signal to be synchronized to the code time modulator. In one embodiment, the code time modulator uses the code signal to modulate the periodic timing signal for channelization and smoothing of a final emitted impulse radio signal. The output of the code time modulator is called the coded timing signal.
The coded timing signal is supplied to a subcarrier time modulator for information modulation thereof. Prior impulse systems used non-subcarrier, baseband modulation. In other words, the information itself was used for modulation. In the present invention, however, an information source supplies an information signal to a subcarrier generator and modulator. The information signal can be any type of intelligence, including digital bits representing voice, data, imagery, or the like, analog signals, or complex signals.
The subcarrier generator and modulator of the present invention generates a modulated subcarrier signal that is modulated by the information signal, and supplies the modulated subcarrier signal to the subcarrier time modulator. Thus, the modulated subcarrier signal is used by the subcarrier time modulator to modulate the carrier, which in this case is the coded timing signal. Modulation of the coded timing signal by the subcarrier time modulator generates a modulated, coded timing signal that is sent to an output stage.
The output stage uses the modulated, coded timing signal as a trigger to generate monocycle pulses. In a radio frequency embodiment, the monocycle pulses are sent to a transmit antenna via a transmission line coupled thereto. The monocycle pulses are converted into propagating electromagnetic pulses by the transmit antenna. The emitted signal propagates to an impulse radio receiver through a propagation medium, such as air in a radio frequency embodiment. In the preferred embodiment, the emitted signals are wide-band or ultrawide-band signals. The spectrum of the emitted signals can be modified by filtering of the monocycle pulses. This filtering will cause each monocycle pulse to have more zero crossings in the time domain. In this case, the impulse radio receiver must use a similar waveform in the cross correlator to be efficient.
There are several impulse radio receiver embodiments. Each impulse radio receiver generally comprises a cross correlator, a decode source, a decode timing modulator and adjustable time base and a subcarrier demodulator.
The decode source generates a decode control signal corresponding to the PN code used by an impulse radio transmitter communicating an impulse radio signal. The adjustable time base generates a periodic timing signal that comprises a train of template signal pulses having waveforms substantially equivalent to each pulse of the received (impulse radio) signal.
The decode timing modulator uses the decode control signal to position in time a periodic timing signal to produce a decode signal. The decode signal is thus matched in time to the known PN code of the transmitter so that the received signal can be detected in the cross correlator.
The decode signal is used to produce a template signal having a waveform designed to match the received signal. The template signal is positioned in time according to the known PN code of the transmitter and is then cross correlated with the received signal. Successive cross correlation output signals are integrated to recover the impulse radio signal out of the noise. Once retrieved in this manner, the signal is demodulated to remove the subcarrier and yield the information signal.
The baseband signal is also input to a lowpass filter. A control loop comprising the lowpass filter is used to generate an error signal to provide minor phase adjustments to the adjustable time base to time position the periodic timing signal in relation to the position of the received signal.
In a preferred embodiment, a subcarrier in an impulse radio translates (or shifts) the baseband signals to a higher frequency. The subcarrier generation and modulator generates a signal that is modulated by the information signal by frequency modulation (FM) techniques, amplitude modulation (AM), phase modulation, frequency shift keying (FSK), phase shift keying (PSK), pulsed FM, or the like.
Other non-sinusoidal and/or non-continuous waveforms can also be employed as subcarriers in connection with the present invention. The modulated subcarrier signal is used to time shift the position of the pulses of the coded timing signal or the periodic timing signal. Thus, the signal that triggers the output stage is a train of pulse position modulated pulses. In another embodiment, direct digital modulation using Manchester encoding is employed as a subcarrier. Combination of these subcarrier techniques is also described.
The effect of using the cross correlation function for the modulation transfer function is to cause the output of the receiver to be a non-linear function of the amplitude of the input. For baseband modulation, this is undesirable. However, for subcarriers, such as FM, AM, FSK, PSK and Manchester, the harmonics are filtered thereby eliminating any distortion. Such filtering can not remove harmonics when baseband modulation is used, because the harmonics stay at baseband, and thus the signal is irrecoverable.
The addition of subcarriers also provides added fidelity in the form of more bandwidth and better signal-to-noise, compared to baseband modulation alone. This benefit is attributed to the fact that the subcarrier inherently renders the information more impervious to noise. The subcarrier embodiments provide less signal compression, and lower signal distortion by reducing baseband noise for high reliability voice, data and/or imagery communications.
The linearity requirements for the modulation using the cross correlator are greatly relaxed by using the subcarrier technique of the present invention. The use of a subcarrier for impulse radios also improves harmonic distortion due to a non-linear modulation transfer function, compared to baseband modulation. Modulation transfer characteristics have to be extremely linear in order to successfully transfer low distortion speech or music. This is very difficult to achieve in a non-subcarrier baseband impulse system.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.